The present invention relates to a magnetic recording medium comprising a support having provided thereon a magnetic layer containing a ferromagnetic powder dispersed in a binder.
The magnetic recording medium is widely used as a recording tape, a video tape and a floppy disc. The magnetic recording medium comprises a support having laminating thereon a magnetic layer containing a ferromagnetic powder dispersed in a binder.
The magnetic recording medium is required to have various characteristics, e.g., electromagnetic characteristics, running durability and running performance in a high level. That is, the audio tape for recording and reproducing music is required to have higher reproducing performance of the original sound. Further, the video tape is required to have excellent electromagnetic characteristics, such as excellent reproducing performance of the original image. The magnetic recording medium is required to have such excellent electromagnetic characteristics and, at the same time, good running durability as described above. As an approach for obtaining good running durability, a method of improving the dispersing property of a binder is innovated.
JP-A-11-39639 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) (hereinafter referred to as xe2x80x9cLiterature Axe2x80x9d) discloses a polyester polyurethane comprising an aliphatic dibasic acid and a polyester polyol comprising a specific branched aliphatic diol, and Literature A further discloses that the improvement of the dispersion stability of a magnetic powder, the improvement of output, the prevention of contamination of a head, the inhibition of increment of dropout (DO), and the improvement of long-term storage stability of the magnetic recording medium (the improvement of the xcexc value (friction coefficient) after one week at 60xc2x0 C. under dry condition) can be ensured by using the polyester polyurethane.
Literature A has been filed by the present inventors, which is characterized in that a polyurethane having high strength and high dispersing property is used as the binder, however, there is a problem that sufficient performance cannot be shown when the magnetic recording medium is stored for a long period of time under high temperature high humidity conditions.
In particular, when a tape is stored for a long term under high temperature high humidity conditions, there is a case where the decomposition of the materials in the tape due to a hydrochloric acid gas and the like takes place, e.g., a fatty acid which is generated due to the hydrolysis of an ester-based lubricant migrates to the surface of the magnetic layer and precipitates and crystallizes. Since the tape has been improved in recording density, even a minute impurity greatly affects the performance of the magnetic recording medium.
The object of the present invention is to provide the following magnetic recording media.
(1) A magnetic recording medium having excellent dispersing property, smoothness of the coated film and excellent electro-magnetic characteristics.
(2) A magnetic recording medium which is excellent in long-term storage stability.
The above object of the present invention has been attained by a magnetic recording medium comprising a support having thereon a lower layer containing a nonmagnetic powder or a ferromagnetic powder dispersed in a binder, and at least a magnetic layer containing a ferromagnetic powder dispersed in a binder provided on the lower layer, wherein the binder contains a polyurethane resin obtained from a polyester polyol, a chain extending agent and an organic diisocyanate, and the magnetic layer surface has a C/Fe peak ratio of from 7 to 11 when measured by the Auger electron spectroscopy, wherein the polyurethane is at least one of
(1) a polyurethane wherein the dibasic acid in the polyester polyol comprises an aliphatic dibasic acid, and 70 mol % or more of the diol component comprises a diol having alkyl branched side chain having 2 or more carbon atoms and not having a cyclic structure, and
(2) a polyurethane wherein the dibasic acid in the polyester polyol comprises an aliphatic dibasic acid, 70 mol % or more of the diol component comprises an alkyl branched aliphatic diol having 2 or more of the total carbon atoms of branched side chain in one molecule, and the chain extending agent comprises an alkyl branched aliphatic diol having 3 or more of the total carbon atoms of branched side chain in one molecule.
The preferred embodiments of the present invention are as follows.
(1) The magnetic recording medium is characterized in that the magnetic layer and/or the lower layer contain(s) an ester-based lubricant.
(2) The magnetic recording medium is characterized in that the magnetic layer has a thickness of preferably from 0.1 to 0.5 xcexcm, more preferably from 0.1 to 0.3 xcexcm.
(3) The magnetic recording medium is characterized in that the ester-based lubricant has a melting point of 10xc2x0 C. or less.